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Refuel on the Fly

Along with providing water and carbohydrates, sport drinks replace some of the minerals you lose through heavy sweating. Three of those minerals—potassium, magnesium, and chloride—are called electrolytes for a simple reason: Your body needs them to transmit electrical signals from your brain to your muscles. Those signals travel through your body's fluids, which are regulated by another electrolyte, sodium.

If you'll be running or riding continuously for longer than an hour, start replenishing your carbohydrate and electrolyte stores around the 30-minute mark, and every 15 minutes after that, Spano says. You want 30 to 60 grams of carbs for every hour of exertion. So if you tank up with 4 ounces of a sport drink (which usually has about 7 grams of carbs) at 1/4-hour intervals, you'll reach the low end of that range. Eight ounces every 15 minutes and you'll be at the high end.